1. Field of the Invention
This invention relates to printing inks, and more particularly to novel printing inks suitable for an absorbent printing object such as paper and having good drying characteristics and workability in printing.
2. Description of the Prior Art
Various types of printing inks used in printing patterns or designs on various printing objects such as paper and plastic film have been heretofore known.
As a printing ink used for non-absorbent printing objects such as a plastic film, metal and glass, there have been known solvent-evaporation type printing inks (e.g. photogravure ink) which are dried by evaporating the ink solvent contained therein.
For absorbent printing objects typified by paper, there have been used litho printing inks, intaglio printing inks, letterpress printing inks, silkscreen printing inks and so on, which are dried by causing the ink solvent contained therein to be absorbed by the absorbent printing object.
The printing inks as noted above are all used for printing by being applied to the face of the printing plate of a printing machine in accordance with a desired pattern form and transferred onto the printing object such as paper. The ink transferred onto the printing object is fixed thereon through the solvent evaporation, solvent absorption, oxidation polymerization of varnish, and other chemical actions.
Thus, the printing inks after being transferred onto the printing object is desired to dry as quick as possible and has generally contrived to be quickly dried, but it is desirable that the printing ink during preservation or in printing does not easily dry. To satisfy the conflicting requirements, the printing ink has been controlled in drying characteristics.
Though there are some cases where printing operations are continuously carried out for hours, the printing operations in most cases are frequently interrupted even in the daytime and suspended during the night. In such cases, the printing ink applied onto the face of the printing plate or a bracket of the printing machine dries and sets up thereon when the good operation is stopped, because the printing ink has moderate drying characteristics as mentioned above.
When the printing ink dries and is fixed onto the face of the printing, it should be removed at the beginning of the next printing operation in order to obtain satisfactory printed matters.
The work of removing the hardened printing ink on the face of the printing plate is much harder and consumes much time, for example, one to two hours. During the work, the printing operation is obstructed. Furthermore, the work of removing the hardened ink requires many workers. Therefore, it can be said that the conventional printing inks are disadvantageous from the standpoint of economy.
Accordingly, there has been a great need for a printing ink which does not dry during preservation or when the printing operation is in preparation or suspended temporarily, but dry quickly after it is transferred to the face of the printing object.
Because the aforementioned requirements are antipodal, there has not been proposed so far any printing ink capable of fulfilling the requirements.
Thus, the conventional printing inks dry quickly and are hardened when being left on the printing plate or other component members of the printing machine for a long time, but do not easily dry when being applied onto the printing object. Therefore, when printed sheets are placed one over another immediately after the printing is completed, the ink applied onto the upper face of the printed sheet sticks easily to the lower face of the adjacent sheet in contact with the upper face of the printed sheet. Because of this, the printing ink applied onto the printing object is required to be forcibly dried, and a plurality of printed sheets should not be put one upon another just after being printed. In a case that the printed sheets must be placed on top of each other, troublesome work of, for example, spraying anti-sticking powder on the printed surface of the printing sheet is required.